A field bus is an industrial communication system at the lowest control level of an automation system which connects a multiplicity of field devices such as sensors, actuators and drives (actuators) to a control device.
The AS interface (AS-I=Actuator-Sensor Interface) is one possible communication standard for communication between the control device (master) and the field devices (slaves) via such a field bus.
In the case of a field bus designed according to the AS-interface specifications, the master is the only component which has the right to access the field bus without being asked to do so. In this case, the master cyclically interrogates all slaves and interchanges the input and output data with the latter via a serial transmission protocol. The slaves must not and cannot access the field bus of their own accord and must wait until they are requested to do so by the master. A unique address is allocated to each of the slaves for this purpose. According to the current specification (vers. 2.11), a maximum of 62 subscribers can thus be connected to a master in an AS interface.
An unshielded two-core line which is preferably in the form of a ribbon cable and can also be simultaneously used as the power supply for slaves is used as the transmission medium for the field bus. For this purpose, the transmission protocol is modulated onto the voltage supply. Manchester coding and alternating pulse modulation coding (APM coding) are used in this case. Bit times of 6 μs can thus be achieved.
The master needs approximately 150 μs to interrogate each slave. A longer period of time is not currently provided in the AS interface since otherwise the cycle between two interrogations, in which the master always interrogates all 62 slaves in succession, would take too long. A message, via which a slave communicates with the master after the slave has been addressed, consists of 4 bits of useful data in this case. This results in extremely short master call and slave response times in the μs range during communication between the master and the slave.
The AS interface meets the interference immunity requirements for communication in industrial installations, inter alia, by virtue of the fact that, according to the AS-interface specifications, the messages can be repeated up to 6 times before an error message or stoppage of the installation results.
In order to increase the availability of the installation in the case of high interference levels as well, a second communication channel could be set up in parallel with the field bus, which communication channel is used to retransmit the same messages as those transmitted via the field bus. In the case of such redundant communication, it suffices for the respectively addressed subscriber to receive at least one of the two messages. Although such redundant systems then increase the installation availability as a result of the lower failure probability, they have a disadvantage. The disadvantage is that the same requirements in terms of volume of data to be transmitted and transmission speed as those imposed on the AS-interface field bus itself have to be imposed on this second communication channel. This renders such a solution complicated and thus expensive.